Flatsy Share definition Flag Derogatory term used in reference to a girl or woman who has small breasts, aka a "flat chest". Might also be used as "Flatsy Patsy". "Hey, that chicks got nothing" "Yeah, she's a Flatsy" Flatsy by mitb April 6, 2009 0 0 Get merch Get the Flatsy mug. Buy Now — $32.95 See more merch